Retractable sprocket pin drives or mechanisms for print media are well known. Such mechanisms comprise retractable pin sprocket wheels disposed at opposite sides of a cylindrical print platen. These are frequently referred to as tractor paper drives. The pins engage perforated edges of the paper to provide a precision, slip-free paper drive.
The platen and the sprocket wheels are secured to a common shaft which is incrementally rotated by a motor such as a stepper motor. As the sprockets rotate the pins track a cam which does not rotate into the region traversed by the print head and carriage adjacent the print platen, so as not to restrict the printing stroke.
Prior art arrangements such as those described in Pat. Nos. 4,133,613, to Webster and 4,571,104, to Jung et al, provide spring loaded pins to track the cam. Webster utilizes individual helical compression springs around individual pins which stroke in radial slots in the sprocket hub, to bias the inner end of each pin against the cam, while Jung et al equally circumferentially space and correspondingly radially position the sprocket pins on an annular spring. Here also the pins are fitted into radial slots in a sprocket hub. A notch in the side of each pin engages and follows a cam track.
Paper drives such as described by Webster and Jung et al are known as single axis paper drives. Although improvements in paper loading are not discussed by either of these patentees paper insertion in single axis drives presents a unique set of problems because the engagement of the pins takes place below the platen and at best is difficult to see, if it can be seen at all. Frequently the leading edge of the paper engages and stops against the side of a sprocket pin. This establishes misalignment between the sprocket pins and the holes along the edges of the paper, causing paper damage at or near the holes, which negates proper paper feed and can lead to a paper jam.
Misalignment of the sprockets at the opposite ends or sides of the print platen can cause the paper to track across the platen. The corresponding pins in the respective sprockets must lie in the same radial plane, otherwise there is a tendency for the paper to track or travel laterally which interferes with printing or plotting and may damage the paper holes, leading to a paper jam.
Similarly, differences in lateral paper dimensions such as between metric and English paper, paper dimensional changes due to environmental conditions, machine assembly tolerances, etc., must be accommodated to avoid paper damage and to achieve proper operation.